Remote access and retrieval of data and information are becoming more desirable and common in both consumer and business environments. As data and information transfer is becoming more and more voluminous and complex, using traditional data links such as voice-band modems is too slow in speed. For example, the use of the Internet to locate and access information is increasing daily, but the retrieval of typical graphics, video, audio, and other complex data forms is generally unsatisfyingly slow using conventional voice-band modems. In fact, the slow rate of existing dial-up analog modems frustrates users, and commerce and interaction using the Internet would have been even higher were it not for the unacceptable delays associated with present day access technology. The ability to provide such desired services as video on demand, television (including HDTV), video catalogs, remote CD-ROMs, high-speed LAN access, electronic library viewing, etc., are similarly impeded by the lack of high speed connections.
Since the alternatives to copper line technology have proven unsatisfactory, solutions to the high speed access problem have been focused on improving the performance of voice band modems. Voice band modems operate at the subscriber premises end over a 3 kHz voice band lines and transmit signals through the core switching network; the phone company network treats them exactly like voice signals. These modems presently transmit up to 33.6 kbps over a 2-wire telephone line, even though the practical speed only twenty years ago was 1.2 kbps. The improvement in voice band modems over the past 20 years has resulted from significant advances in algorithms, digital signal processing, and semiconductor technology. Because such modems are limited to voice bandwidth (3.0 kHz), the rate is bound by the Shannon limit, around 30 kbps. A V.34 modem, for example, achieves 10 bits per Hertz of bandwidth, a figure that approaches the theoretical Shannon limits.
There is a considerable amount of bandwidth available in copper lines, however, that has gone unused by voice band modems, and this is why a proposal known as Asymmetric Digital Subscriber Line (ADSL) was suggested in the industry as a high-speed protocol/connection alternative. The practical limits on data rate in conventional telephone line lengths (of 24 gauge twisted pair) vary from 1.544 Mbps for an 18,000 foot connection, to 51.840 Mbps for a 1,000 foot connection. Since a large proportion of current telephone subscribers fall within the 18,000 foot coverage range, ADSL can make the current copper wire act like a much "bigger pipe" for sending computer bits and digital information like movies and TV channels), while still carrying the voice traffic. For example, an ADSL modem can carry information 200 times faster than the typical voice band modem used today.
ADSL is "asymmetric" in that more data goes downstream (to the subscriber) than upstream (back from the subscriber). The reason for this is a combination of cost, demand, and performance. For example, twisted pair wiring coupling increases with the frequency of the signal. If symmetric signals in many pairs are used within a cable, the data rate and line length become significantly limited by the coupling noise. Since the preponderance of target applications for digital subscriber services is asymmetric, asymmetric bit rate is not perceived to be a serious limitation at this time. Therefore, the ADSL standard proposes up to 6 Mbps for downstream, and up to 640 kbps for upstream. For example, video on demand, home shopping, Internet access, remote LAN access, multimedia access, and specialized PC services all feature high data rate demands downstream, to the subscriber, but relatively low data rates demands upstream. The principal advantage is that all of the high speed data operations take place in a frequency band above the voice band, leaving Plain Old Telephone Service (POTS) service independent and undisturbed, even if an ADSL modem fails. ADSL further provides an economical solution for transmission of high bandwidth information over existing copper line infrastructures.
Specifically, the proposed standard for ADSL divides the available transmission bandwidth into two parts. At the lower 4 kHz band, ordinary (POTS) is provided. The bulk of the rest bandwidth in the range from 4 kHz to about 1 MHz is for data transmission in the downstream direction, which is defined to be from the exchange to the subscriber. The upstream control channel uses a 160 kHz band in between. The signals in each channel can be extracted with an appropriate band-pass filter.
A DMT implementation of ADSL uses the entire available 1 MHz range of a copper phone line. It merely splits the signal into 255 separate channels, and each 4 kHz channel can be made to provide a bit rate up to the best present day voice band (33.6 kbs) modems. This results essentially in overall performance which is equivalent to around two hundred V.34 modems used in parallel on the same line. Because each channel can be configured to a different bit rate according to the channel characteristics, it can be seen that DMT is inherently "rate-adaptive" and extremely flexible for interfacing with different subscriber equipment and line conditions.
A number of problems arise, however, in attempting to implement a full scale ADSL transceiver cost-effectively.
First, to achieve this high bit rate transmission over existing telephone subscriber loops, advanced analog front end (AFE) devices, complicated digital signal processing techniques, and high speed complex digital designs are required. As a result, this pushes current technology limits and imposes both high cost and power consumption. For example, AFE devices in modem applications provide the interface between analog wave forms and digital samples for digital hardware/software processing. In high speed modem technologies such as ADSL, AFE devices need to operate at a very high sampling rate and high accuracy. For example, the DMT technology has a spectrum of 1 MHz and requires sampling above 50 MHz if a sigma-delta analog-to-digital (ADC) method is used. This thus requires the state-of-art ADC technology and imposes a high cost for end users.
Second, the time domain signal in ADSL/DMT transmissions is a summation of a large number of carriers modulated by quadrature amplitude modulation (QAM). This typically results in a large peak-to-peak deviation. As a result, even though a high speed AFE is made possible, a large dynamic range and high resolution AFE is required at the same time to minimize quantization errors.
Third, in addition to the high sampling rate and resolution requirement for ADSL AFEs, the other hardware and software in ADSL environment also needs to operate at a much higher speed than current conventional modem counterparts. For example, to implement the DMT technology in software, a custom and dedicated digital signal process (DSP) of a power of several hundred MIPS (millions instructions per second) is required to process many components such as error encoding and decoding, spectrum transforms, timing synchronization, etc. As with the AFE part of the system, this high speed requirement for the signal processing portion of ADSL also results in less flexible, high component costs.
Fourth, requiring a communications device (such as a modem) to fully support the total throughput of a standard such as ADSL may be inefficient in some cases, since many prospective users of high-speed data links may not need to use all the available bandwidth provided by such standards. It is generally more preferable therefore to permit users to throttle or scale the data throughput in a manner they can control, based on their particular application needs, hardware cost budget, etc. For example, a fill-scale ADSL system may have the performance level of 200 times conventional V.34 modems, but it is apparent that even a performance improvement of 10-20 times than present day available analog modems would be sufficient for many consumer applications, such as Internet access and similar uses. Thus, unlike conventional analog modems, which are available in various speeds varying generally from 14.4 to 56 Kbps, there are no known ADSL modems which offer scalable performance levels to users.
Fifth, in addition to the implementation challenge, the T1E1.4 AD SL standard does not specify the system interface and user model. Although various high level interface to support T1/E1, ATM, etc. have been described, system integration with high level protocols such as TCP/IP and interface with computer operating systems have not yet been defined. As a result, there is uncertainty how existing and future modem-based applications can work with the ADSL technology. For example, when users run an Internet application which sends and receives data to and from an Internet service provider (ISP), a mutually agreed protocol is required to set up a call and transfer data. Possible protocols available at various levels include ATM (asynchronous transfer mode), TCP/IP, ISDN, and current modem AT commands. Either one of these or a possibly new protocol needs to be defined to facilitate the adoption of ADSL technology.